Don’t worry. This is not nearly as gross as a frog. Besides, in my opinion, this is a whole lot more interesting. Read on, and you’ll discover how we transformed five guards into a roomful.
To begin with, we shot some nice video of our villain (me!) with some space around him. Here’s a frame from that:

Next, we broke out the good old blue screen, dressed up five of our seven people to be Norse guards, and shot some video. These are some resulting frames, before keying out the blue screen:


Okay, we’ve got the video that we need. Time to put it together. I launched my favorite video editing software (Final Cut Express) and got to work.
I laid down the villain shot as the background, then added one layer of Norse dudes over the top. A bit of color correction plus some chroma keying, and I had this result:

Not bad, but still a bit unimpressive. Besides, one of those guards looks oddly familiar. Unless she’s got an identical twin serving in this army, I’m not mistaken in thinking that she’s in there twice. Time to cover her up.
I took another clip of blue screen guards (we did three arrangements with each actor switching costumes for each setup), mirrored it to face the other way, and placed that clip on top of the other two. Now we have two rows of guards, no duplicates (except that I’m playing the villain and a guard, but I’ve got a helmet covering my face, so you can’t really tell), and a room full of more people than we had actors. Here’s a frame from the final video:

Mission accomplished!
Wow. This is one of the most awesome things I have ever seen. I am an aspiring filmmaker also, but I haven’t had the opportunity yet to do films that require special effects, mostly because I have a one person cast. But I hope I’ll get a chance sometime to do something as cool as this!
Just make sure that if you try this, you light your bluescreen better than we did. I had a really hard time getting this one right, and even now it has problems.